The present invention generally relates to apparatus and methods for cooling foil thrust bearings and, more specifically, to apparatus and methods for improving the transfer of heat through the foil and into the cooling fluid of foil thrust bearings.
Foil thrust bearings are very attractive for high speed rotating machinery, such as, for example, a turboalternator-generator, turbocompressors, and motor driven compressors. One of the benefits of such bearings is that they do not require oil-based lubrication and the corresponding high maintenance costs generally attributable to oil-based lubricating systems. Instead of using oil, foil thrust bearings generally use readily available ambient atmosphere air as the lubricating and cooling fluid, although other lubricating fluids, including methane, water, or gaseous or liquid hydrogen, nitrogen, or oxygen, may be used.
Foil thrust bearings generally comprise two members which rotate with respect to each other and which are positioned such that a predetermined space between them is filled with the lubricating and cooling fluid. Foils (or thin sheets of compliant material) disposed in the space are deflected by the hydrodynamic film forces between the adjacent bearing surfaces. The foils enhance the hydrodynamic characteristics of the bearing, provide support between the bearing and the runner, accommodate eccentricity between the relatively movable members, and also provide a cushioning and dampening effect.
To properly position the foils between the movable bearing members, it is known in the art to mount a plurality of individually spaced foils on a foil or thrust bearing disk and position the disk on one of the bearing members. Another similar common practice has been to provide separate compliant stiffener elements or undersprings beneath the foils to supply the requisite compliance.
Compliant hydrodynamic bearings are a well known class of foil thrust bearings, and have been praised for their high rotor speed capability as well as their ability to tolerate rotor/bearing misalignment and thermal distortion. These capabilities, along with the ability to use the machine""s own process fluid as the bearing lubricant, have made compliant hydrodynamic bearings an attractive alternative for use in high-speed turbomachinery applications.
Hydrodynamic bearings support bearing loads by generating fluid pressure through viscous shear of the bearing lubricant into a converging geometry or xe2x80x9cwedgexe2x80x9d bounded by the bearing surfaces. This shearing action is provided by and is in the direction of the relative motion of the bearing surfaces. This shearing action also generates heat. The rate of heat generation is proportional to the dynamic viscosity and the square of the relative surface speed and inversely proportional to the film thickness. Thus, heavily loaded bearings having thin hydrodynamic film thickness and operating at high surface speeds produce a significant amount of heat that must be removed in order to avoid excessive bearing temperatures.
Many compliant bearing inventors have disclosed methods to improve bearing load capacity by optimizing the shape of the bearing""s hydrodynamic fluid film. An example of this approach is U.S. Pat. No. 5,318,366 to Nadjafi, et. al. which teaches use of variable width spring xe2x80x9cfingersxe2x80x9d in order to tailor spring stiffness as a means to optimize hydrodynamic wedge shape and provide high load capability. However, bearing cooling schemes have been given much less attention although thermal distortion of the bearing components and temperature limitations of fluid foil coatings do limit the load supporting capacity of these bearings. Thus, bearing cooling is an important design consideration.
One prior art example addressing a bearing cooling scheme is U.S. Pat. No. 4,247,155 to Fortmann. This invention introduces a single piece top foil with perforations to channel cooling flow into the hydrodynamic wedge, and to reduce the bending stiffness of the single piece top foil so as to facilitate the creation of a plurality of bearing pads under the hydrodynamic pressure load. In this application, cooling flow through the spring structure travels in essentially straight-through paths, and there is no attempt made to tailor cooling air flow such as to maximize convective heat transfer.
In many applications, as in an air cycle machine, air is the process fluid and is also the bearing lubricant. In this case, the bearings are generally cooled by bleeding pressurized air from the air cycle machine""s compressor outlet, and channeling it through the bearing""s spring support structure. This cooling flow is predominately radial for a thrust bearing. Some of this flow enters the bearing""s hydrodynamic film, replenishing lubricant lost to bearing side leakage. Side leakage is that portion of the lubricant that leaks out of the axial ends of the journal bearing (or out of the inside diameter and outside diameter of a thrust bearing) as it flows into the converging hydrodynamic wedge. Some of the bearing heat is removed as this heated fluid mixes with the cooling flow and is carried along downstream of the bearing. However, side leakage only removes a small percentage of the heat, and most of the heat is removed through convection between the underside of the xe2x80x9chotxe2x80x9d fluid foil (i.e. the surface facing away from the hydrodynamic film) and the cooling flow through the support structure. Further, the cooling air that comes in direct contact with the underside of the hot foil provides the most efficient convection heat transfer. Bearing cooling can be increased by simply increasing cross sectional cooling flow area. However, this is inefficient in that it requires more air than necessary to be bled off the compressor and the overall efficiency of the turbomachine will be degraded.
As can be seen, there is a need for an improved foil and thrust bearing that makes use of proven principals from heat exchanger design and applies them to the bearing structure in order to maximize convective heat transfer and minimize the rate of cooling flow that is required to remove the heat generated in the bearings.
In one aspect of the present invention, there is disclosed a foil thrust bearing, comprising a thrust runner and a thrust plate arranged for relative rotation with respect to one another; a thrust bearing disk operably disposed adjacent said thrust runner; an underspring element operably disposed adjacent said thrust plate; a turbulence generating disk operably disposed between said thrust bearing disk and said underspring element; a plurality of turbulator elements integral to said turbulence generating disk, positioned on the inner annular ring edge of said turbulence generating disk, and directed radially toward the center of said turbulence generating disk; and cooling fluid flow directed from the outside diameter to the inside diameter of said foil thrust bearing and along said turbulator elements of said turbulence generating disk.
In another aspect of the present invention, there is disclosed a foil thrust bearing, comprising a thrust runner and a thrust plate; a thrust bearing disk adjacent said thrust runner; an underspring element adjacent said thrust plate; a turbulence generating disk between said thrust bearing disk and said underspring element, said turbulence generating disk comprising a plurality of turbulator elements directed radially toward the center; and, cooling fluid flow directed radially along said turbulator elements of said turbulence generating disk.
In another aspect of the present invention, there is disclosed a system for cooling foil thrust bearings for high speed rotating machinery, comprising a foil thrust bearing further comprising a thrust runner and a thrust plate arranged for relative rotation with respect to one another; a thrust bearing disk disposed adjacent said thrust runner; an underspring element disposed adjacent said thrust plate; a turbulence generating disk, disposed between said thrust bearing disk and said underspring element, and comprising a plurality of turbulator elements positioned on the inner annular ring edge and directed radially toward the center of said turbulence generating disk; and, turbulent cooling fluid flow along said turbulator elements and directed to said foil thrust bearing.
In another aspect of the present invention, there is disclosed a method for cooling foil thrust bearings for high speed rotating machinery, comprising the steps of: rotating a thrust runner in relation to a thrust plate; inserting a thrust bearing disk and an underspring element between said rotating thrust runner and thrust plate; inserting, between said thrust bearing disk and said underspring element, a turbulence generating disk comprised of turbulator elements positioned on the inner annular ring edge and directed radially toward the center of said turbulence generating disk; generating cooling fluid pressure by bleeding pressurized fluid from said high speed rotating machinery; and, generating turbulent cooling fluid flow by directing said cooling fluid flow through said turbulator elements of said turbulence generating disk.
In yet one further aspect of the present invention, there is disclosed a method for cooling foil thrust bearings for rotating machinery, comprising the steps of: rotating a thrust runner in relation to a thrust plate; inserting a thrust bearing disk, a turbulence generating disk, and an underspring element between said rotating thrust runner and thrust plate; generating cooling fluid pressure by bleeding pressurized fluid from said rotating machinery; and, generating turbulent cooling fluid flow by directing said cooling fluid flow through said turbulence generating disk.